Powerplant and related control system and method

ABSTRACT

A hydrogen fueled powerplant including an internal combustion engine that drives a motor-generator, and has a two-stage turbocharger, for an aircraft. A control system controls the operation of the motor-generator to maintain the engine at a speed selected based on controlling the engine equivalence ratio. The control system controls an afterburner, an intercooler and an aftercooler to maximize powerplant efficiency. The afterburner also adds power to the turbochargers during high-altitude restarts. The turbochargers also include motor-generators that extract excess power from the exhaust.

This application claims the benefit of U.S. provisional Application No.61/194,048, filed Sep. 23, 2008, which is incorporated herein byreference for all purposes.

The present invention relates generally to a hydrogen powerplant and,more particularly, to a powerplant having a hydrogen combustion engine,a turbocharger and an afterburner.

BACKGROUND OF THE INVENTION

High altitude long endurance aircraft require extremely efficientdesigns. Hydrogen-powered aircraft have been previously suggested tolimit fuel weight and thereby maximize endurance. Regardless of theselected fuel, an oxidizer must be provided, and if ambient air is to bethe oxidizer, it typically must be compressed at high altitudes. Whenhydrogen is the fuel, a significant amount of compressed air may remainunburned by an engine, and thus the energy of compression may be wastedfor that unburned portion of the air.

It has been suggested that the use of electric motors for propulsion maybe beneficial for high altitude long endurance aircraft. If a highaltitude long endurance aircraft is to operate electric motors, asignificant electrical power generation system is necessary. Suchsystems must be able to quickly adapt to changing power requirements,but complex systems with heavy components are detrimental, in that theylimit the payload (or duration) of the aircraft, and typically havelower reliability. Simpler systems, on the other hand, may be limited intheir ability to adapt to rapidly changing power requirements.

Conventional turbocharged engines are usually designed to control boostpressure using a wastegate. A wastegate is a controllable valve in theexhaust stream that bypasses some fraction of exhaust gases pass aturbocharger, thereby providing control over the turbocharger speed andresulting compressor boost. The continuous use of a wastegate canprovide for the prompt ability to boost power (by closing thewastegate), but use of a wastegate wastes some of the energy thatotherwise would be recoverable from the exhaust. In a high-altitudehydrogen powerplant, it is important to optimize efficiency, so aturbocharger system is typically set out to operate normally with zerowastegate flow. Because of this, it is not possible to increase boost byfurther using a wastegate.

Accordingly, there has existed a need for an aircraft powerplant thatcan provide highly efficient power with high reliability, while allowingfor rapid changes in operating levels. Preferred embodiments of thepresent invention satisfy these and other needs, and provide furtherrelated advantages.

SUMMARY OF THE INVENTION

In various embodiments, the present invention solves some or all of theneeds mentioned above, offering a powerplant that can provide efficientpower with high reliability, while allowing for rapid changes inoperating levels and addressing other related issues, such as enginestartup and engine temperature limits.

The powerplant of present invention includes a combustion engineconfigured to produce motive force, a generator configured to generatepower from the motive force of the engine. The engine has an inlet andan outlet. The generator is configured to apply a variable-level motiveforce to the engine. The powerplant further includes a control systemconfigured to control the motive force applied to the engine by thegenerator. It is adapted to control the generator such that thegenerator maintains the engine speed at a selected, substantiallyconstant speed during steady state engine operation, and to vary theengine speed during transient conditions based on maintaining anacceptable equivalence ratio and maximizing overall powerplantefficiency. Advantageously, this provides for the engine to be operatedat selected speeds that promote maximum efficiency and reliability. Theengine is further configured without a throttle, which improves both itsweight efficiency and the reliability of the engine.

The powerplant further includes a turbocharger having a turbine and acompressor, and an afterburner in the exhaust stream intermediate theengine and the turbine. The afterburner is configured to reactadditional reactants in the exhaust stream, and the control system isconfigured to control the operation of the turbocharger by controllingthe amount of additional reactants reacted in the exhaust stream.Advantageously, the resulting additional energy provides for theafterburner to partially or completely power the turbocharger, such asduring engine startup, transitions in power levels, and the like. Theresulting increased air flow rates can also be used to keep fuel levelslean, and thereby limit engine exhaust temperatures.

Advantageously, some aspects of the invention provide an expansion ofachievable operating points without the need for a wastegate, theability to restart the engine at high altitude, and the recovery ofenergy content of any unburned hydrogen in the exhaust of the powerplantcore.

Other features and advantages of the invention will become apparent fromthe following detailed description of the preferred embodiments, takenwith the accompanying drawings, which illustrate, by way of example, theprinciples of the invention. The detailed description of particularpreferred embodiments, as set out below to enable one to build and usean embodiment of the invention, are not intended to limit the enumeratedclaims, but rather, they are intended to serve as particular examples ofthe claimed invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system layout of a first embodiment of a powerplant underthe invention.

FIG. 2 is an elevation view of an engine and a generator of theembodiment depicted in FIG. 1.

FIG. 3 is an system layout of the connections of a control system of theembodiment depicted in FIG. 1.

FIG. 4 is a first method under the invention, using the embodimentdepicted in FIG. 1.

FIG. 5 is a second method under the invention, using the embodimentdepicted in FIG. 1.

FIG. 6 is a third method under the invention, using the embodimentdepicted in FIG. 1.

FIG. 7 is a system layout of another embodiment of a powerplant underthe invention.

FIG. 8 is a system layout of yet another embodiment of a powerplantunder the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention summarized above and defined by the enumerated claims maybe better understood by referring to the following detailed description,which should be read with the accompanying drawings. This detaileddescription of particular preferred embodiments of the invention, setout below to enable one to build and use particular implementations ofthe invention, is not intended to limit the enumerated claims, butrather, it is intended to provide particular examples of them.

Typical embodiments of the present invention reside in a powerplant fora high-altitude long-endurance aircraft. The powerplant includes anengine (i.e., a machine that converts energy into mechanical force ormotion) and a primary motor-generator configured to generate electricityusing the motive force of the engine, to provide motive force to spinthe engine, and to provide motive force to the engine to regulate thespeed with which the engine spins. The engine uses a first reactant anda gaseous stream of a second reactant, which may respectively be a fuelsuch as hydrogen from a fuel source (e.g., a hydrogen tank), and agaseous stream of an oxidizer such as air from an oxidizer source suchas the atmosphere. Other fuels, oxidizers, fuel sources and oxidizersources are also contemplated within the scope of the invention.

System Configuration

More particularly, with reference to FIGS. 1-3, the powerplant of thefirst embodiment includes a power generation system including aninternal combustion piston engine 101 that drives a primarymotor-generator 103 (hereinafter referred to as “the primarygenerator”). The engine is configured to produce rotational motiveforce, and has an inlet 105 and an exhaust outlet 107, as is commonlyknown. Optionally, the engine and primary generator may be integral inthat they operate using a single, common shaft 109 that extends throughboth and serves as both an engine crankshaft and a generator shaftcarrying a rotor 111. Optionally, the rotor may be sized to serve as aflywheel for the engine. Other types of engines are also contemplatedwithin the scope of the invention.

The primary generator 103 is configured to generate electric power fromthe motive force of the engine 101 (e.g., from the rotation of thecommon shaft 109). The primary generator is further configured to applya variable-level of motive force to the shaft of the engine. This motiveforce applied to the engine can actively spin the (shaft of the) engine,such as to start the engine, or can be used to control the speed withwhich the engine is running.

The powerplant also includes a control system 113 configured to controlthe operation of the powerplant over an envelope of operating conditions(e.g., over a range of power generation requirements and over a range ofambient conditions that can range from sea-level temperatures andpressures to stratospheric conditions). In a first aspect, the controlsystem is configured to control the variable-level of motive forceapplied to the engine 101 by the primary generator 103 as the engine isdriving the primary generator. In a second aspect, the control system isconfigured to energize the primary (motor-) generator as an electricmotor to drive the engine.

More particularly, the control system 113 is configured to control themotive force applied by the primary generator 103 to the engine 101based on a calculation of the engine equivalence ratio. Moreparticularly, it is configured to control the motive force such that theprimary generator controls and varies the engine speed during normalengine operation, over the envelope of operating conditions, such thatthe equivalence ratio is maintained within acceptable limits (which willtypically depend on the type of fuel being used) and the overallpowerplant efficiency is maximized. During both steady state conditionsand transient conditions (e.g., during changes between different powergeneration levels or changes in ambient conditions) the equivalenceratio is selected to maximize powerplant efficiency, but will balance apreference for an optimal equivalence ratio with the thermal andmechanical limits of the various parts of the powerplant.

The variation of motive force by the generator is typically done byvarying the magnetic field generated by field coils of the primarygenerator (e.g., by varying the electric power level that generates amagnetic field for the primary generator), but could also beaccomplished by varying the number of active windings in the armature.For a given engine speed, as the motive force applied by the primarygenerator to the engine is increased, the electric power generated bythe primary generator is also increased. Thus, when the engine powerlevel increases, the primary generator increases its motive force tomaintain engine speed, and thereby increases the power level itgenerates.

At high altitudes the ambient pressure is low, and to use ambient air asa source of gaseous engine oxidizer, a substantial amount of compressionmay be required to provide enough oxygen for combustion in the engine101. To provide for the airflow requirements to support combustion, thepresent embodiment of a powerplant includes a compression systemconfigured to compress gaseous engine oxidizer. More particularly, thecompression system includes a ram-air scoop 119, a first-stageturbocharger including a first-stage compressor 121 and a first-stageturbine 123, an intercooler 125, a second-stage turbocharger including asecond-stage compressor 131 and a second-stage turbine 133, anaftercooler 135, and an afterburner 137.

In operation, the first-stage compressor 121 receives air from theram-air scoop 119, which is configured to use the flight speed of theaircraft to aid in efficiently gathering air. The first-stage compressoris driven in rotation to compress the gathered air (a gaseous engineoxidizer for the engine 101) from the ram-air scoop and generate astream of once-compressed air that has been heated by the action of thefirst-stage compressor. The intercooler 125 then cools theonce-compressed air to lower its temperature in preparation for furthercompression.

The second-stage compressor 131 receives the stream of once-cooled,once-compressed air from the intercooler 125, and is driven in rotationto further compress it to generate a stream of twice-compressed air thathas been heated again, this time by the action of the second-stagecompressor. The aftercooler 135 then cools the twice-compressed air tolower its temperature in preparation for induction onto the inlet 105and then combustion. This decrease in engine air intake temperatureprovides a denser intake charge to the engine 101 and allows more airand fuel to be combusted per engine cycle, increasing the output of theengine. Thus, the powerplant includes two serial (i.e., located inseries) compressors configured to compress a gaseous engine oxidizer forthe engine, an intercooler configured to cool the gaseous engineoxidizer intermediate the two compressors, and an aftercooler configuredto cool the gaseous engine oxidizer after compression by both of the twocompressors.

Because aircraft systems must balance a variety of factors, includingreliability, weight and energy efficiency, it is typically desirable tominimize the amount of structure, the number of moving parts, andsystems that cause inefficient energy losses. In the present embodiment,the engine 101 does not use, and is not provided with, a throttle (i.e.,a controllable inlet obstruction that causes a variable pressure drop toa stream of a gaseous inlet reactant such as the twice-compressed andtwice-cooled ambient air).

More particularly, the engine 101 is configured with an inlet passagewayextending serially from the ram-air scoop 119, through the first-stagecompressor 121, the intercooler 125, the second-stage compressor 131,and the aftercooler 135 to terminate at the engine inlet 105. This inletpassageway is configured to pass a stream of gaseous engine oxidizerwithout limitation from a controllable obstruction (e.g., a throttle)that would cause a reduction in pressure of the gaseous engine oxidizerstream (i.e., creating a pressure drop from an upstream side of theobstruction to a downstream side of the obstruction).

It should be noted that a distinction is being drawn here between acontrollable obstruction (i.e., a throttle), and an obstruction that isconfigured to at all time minimize its resistance to flow (e.g., pipingbetween components, or the coolers, which are configured to minimizetheir flow resistance while maximizing their cooling of the stream ofair). Moreover, the inlet passageway is configured to always provide thestream of gaseous engine oxidizer to the engine inlet 105 atsubstantially the pressure level established by the second-stagecompressor 131, in that it differs only by a small amount caused by theaftercooler 135 and piping, which varies only as a function of aconstant loss coefficient (and of fluid velocity), rather than beingcharacterized by a variable loss coefficient.

After the fuel and oxidizer are mixed, combustion occurs, and the engineconverts some of the energy of the combustion into the motive force ofthe engine. The combusted fuel and oxidizer are then passed out theexhaust outlet 107 into an exhaust passageway. The exhaust passagewayextends serially from the exhaust outlet, through the afterburner 137,the second-stage turbine 133 and the first-stage turbine 123 beforeending at a port configured to expel the exhaust into the ambientatmosphere. Thus, the afterburner is located in the exhaust streamintermediate the engine and the turbines.

Unlike a gasoline engine, which normally operates in a stoichiometricmode in which the fuel reacts all of the oxygen in the air, the hydrogenengine 101 of the present embodiment typically operates in a mode inwhich the oxygen reacts substantially all of the fuel, and some oxygenremains present. Thus, the engine is a hydrogen-fueled engine configuredto run with an equivalence ratio of less than one. With excess oxygen inthe exhaust from the powerplant core (i.e., the engine), there is thepotential to burn additional fuel in the exhaust. This afterburningcapability in an exhaust stream has been employed previously to provideadditional propulsion for an aircraft takeoff. This was done by burningfuel in an exhaust stream that was directed through a power-takeoffturbine mechanically coupled to a propeller.

In operation, the afterburner of the present embodiment receives exhaustair directly from the engine 101. As discussed above, typically thisexhaust will principally contain some gaseous second reactant in theform of an oxidizer, along with combustion exhaust product.Nevertheless, an exhaust containing a fuel and exhaust product, orcontaining fuel, oxidizer and possibly an exhaust product (in the caseof incomplete engine combustion) are contemplated within the scope ofthe invention.

The afterburner 137 is configured to react additional reactants in theexhaust stream, and more particularly, it includes an afterburner fuelinjector 139 configured to inject a third reactant into the exhauststream. In this embodiment the third reactant is a hydrogen fuel (andthe injector is a hydrogen fuel injector). Typically the third reactantwill be a fuel of the same type as the first reactant (i.e., the fuelreacted in the engine 101, e.g., hydrogen), though it could be one ofanother type, or it could be an oxidizer.

The afterburner 137 may be of any known type, but will typically be acatalytic burner to maximize combustion of the remaining reactants untilthe supply of one reactant is substantially exhausted. The diameter ofthe catalyst bed in the afterburner is selected such that there is aninconsequential pressure drop across the afterburner. The afterburnerinjector 139 and an optional mixer (not shown) is typically placedseveral diameters upstream of the burner. A means of controlling themass flow of the injected reactant is provided. This may typically be ahydrogen mass flow controller or one or more pulse-width modulated fuelinjectors.

When fuel and oxidizer are being reacted in the afterburner 137,additional energy is added to the exhaust stream. From the afterburnerthe exhaust stream is directed through the second-stage turbine 133,which is driven in rotation by the exhaust stream, and thus removesenergy from the exhaust stream to drive the second-stage compressor 131in rotation.

Likewise, from the second-stage turbine 133, the exhaust stream isdirected through the first-stage turbine 123, which is driven inrotation by the exhaust stream, and thus removes energy from the exhauststream to drive the first-stage compressor 121 in rotation. Optionally,an additional or alternative afterburner 141 (and optionally a relatedafterburner fuel injector 143) could be located along the exhaustpassageway intermediate the second- and first-stage turbines 133, 123.While it is possible that this is a less efficient use of anafterburner, it could both avoid exposing the second-stage turbine 133to excessive temperatures, and provide a more direct and controllableinfluence over the operation first-stage turbine 123. It also may be aless expensive device, as it would not need to operate in the hightemperatures of the primary afterburner 137.

Using the catalytic afterburner 137, it is possible to add energy to theexhaust to boost power when an increase in compression is needed.Because the burning of fuel in the afterburner to power the turbinesdoes not add directly to the motive force the engine applies to theprimary generator 103, the injection of reactant into the afterburnerwill typically only be done for only short, selected periods of time. Itis better to operate with lower efficiency during such short periodsusing the afterburner, than to operate with lower inefficiency for longperiods of time, such as would occur by using a frequently openwastegate that could be closed to provide momentary power boosts.

A turbocharger system for a high altitude powerplant will typically bequite large and can have significant heat capacity, which takes asignificant amount of time to warm up to operating temperature after anormal engine start. One such time for afterburner operation may beduring a system start. By reacting additional hydrogen in the exhaustduring a warm-up operation, the afterburner 137 can generate theadditional heat needed for warming the turbines 123, 133. This cansignificantly accelerate the operation of warming the turbochargers upto operating temperatures, avoiding excessive operation at coldtemperatures. Thus, the system includes a control system configured tocontrol the operation of the afterburner to power the turbine during asystem cold start such that the warming of the turbine to operationaltemperatures is accelerated over a start without the afterburner. Thisfeature may be augmented with the use of turbine temperature sensors, orwith information on the typical time for the turbine to come up tospeed.

An exception to the generally short times of afterburner operation maybe during engine idle while at high altitude. Engine idle typicallyoccurs with only a minimum amount of fuel use, and thus has only aminimum amount of energy available in the exhaust. Nevertheless, at highaltitude it may take a significant amount of energy to compress even thesmall amount of air needed for idling the engine 101. Thus, the engineexhaust at idle may not have enough energy to keep the turbochargerspowered adequately to provide even the minimal compression needed atidle. Using the afterburner 137 to provide additional energy in theexhaust (typically by reacting additional fuel) can provide theadditional energy needed by the turbines to maintain adequatecompression for engine operation.

If at any time the engine 101 incompletely reacts the reactants (i.e.,if the exhaust contains more than trace amounts of both a fuel and anoxidizer), the catalytic afterburner 137 can also react these reactantswithout any additional efficiency losses (e.g., on spent fuel or energylost to additional compression). For example, at very lean operatingpoints, a hydrogen internal combustion engine might not completely burnall of the injected hydrogen. The catalytic afterburner in the exhaustpassageway will react the unburned hydrogen, capturing energy as heatthat would otherwise have been lost. This captured heat energy may bepartially converted to mechanical energy in the first- and second-stageturbines 123, 133, increasing their ability to drive the first- andsecond-stage compressors 121, 131 in rotation.

The powerplant is designed for efficient operation at typical operatingconditions, such as those it will experience during the majority offlight conditions. While it is preferable that the system operates witha minimum of energy loss at all times, there may be limited time periodswhen the compression capability of the system detrimentally overwhelmsthe engine requirements. One such time might be when the powerplant isin a low-power mode. This could, for example, occur during a descent(characterized by minimum thrust power requirements) when no payloadpower is needed. Another such time may be during a rapid change in thefuel-flow rate.

To accommodate the operation of the powerplant during these limitedtimes, the powerplant is provided with one or more energy bleed devices.A first type of such device is configured to lower the energy availablein the exhaust stream to drive the turbines 123, 133. A first suchdevice of this type is a low-pressure wastegate 161 configured to bleedoff some of the pressurized exhaust prior to it being used to drive thefirst-stage (i.e., low pressure) turbine 123. Another such device ofthis type is a high-pressure wastegate 163 configured to bleed some ofthe pressurized exhaust around the second-stage (i.e., high pressure)turbine 133. The high-pressure wastegate can alternatively be configuredto externally bleed the pressure out of the system, and thereby lowerthe energy available to both the first- and second-stage turbines.

A second type of energy bleed device is configured to bleed off thecompressed gaseous stream prior to its being ingested by the engine 101.A first such device of this type is an engine bypass 165 configured tobleed off some of the compressed gaseous stream into the engine exhaust.It can be located either prior to or after the aftercooler 135. Anothersuch device of this type is an inlet blow-off valve 167 configured tobleed off some of the compressed gaseous stream immediately prior to itbeing received by the inlet 105, or alternatively, prior to theaftercooler. As was previously noted any one, two, three or four ofthese in combination are contemplated within the scope of the invention,and the number, size and types used may vary depending on thecharacteristics of a particular design. For example, for some designsonly the inlet blow-off valve 167 may be needed.

Variation in Power Generation Levels

As noted above, the primary generator 103 is controlled to maintain theengine (and primary generator rotor) speed at speeds appropriate tocontrol the equivalence ratio to acceptable levels (even throughchanging fuel flow requirements). Nevertheless, the aircraft willrequire differing levels of electrical power generation depending on theflight conditions, the flight plan, and the varying demands of theaircraft systems and the aircraft payload. The primary generatorgenerates various levels of electrical power at a power generation levelrelated to a rate at which fuel is provided to the engine 101 (andlikewise, to the level of motive force the primary generator applies tothe engine to keep it at an acceptable equivalence ratio).

The greater the fuel level supplied to the engine 101, the greater themotive force supplied by the engine, and the more resistance to rotationthe primary generator 103 applies to maintain a constant engine androtor speed. This increase in resistance is the natural result ofincreased power generation, and thus when the rate of fuel fed to theengine increases, the level of electrical power generation alsoincreases. Thus, within the operating envelope of the engine and primarygenerator, a wide range of power generation levels can be achieved, eachwith an associated fuel-flow rate to the engine.

Nevertheless, transition between two far-apart power levels can beproblematic, as the steady state operating conditions of the engine 101,turbochargers and coolers 125, 135 can be substantially different, andeach device has a limited range of operation in which it is efficientand safe for operation. For example, the fuel to oxidizer ratio of theengine needs to be maintained within a safe operating range—even duringa transition. A rapid change in fuel-flow rate that is not accompaniedby a proportional change in air flow rate could cause an engine misfireor over temperature event.

Likewise, for any given pressure boost ratio, each turbocharger has acharacteristic range of corrected air flow rates for which a given levelof efficiency (i.e., a percentage of turbo shaft power that converts toactual air compression) can be maintained. These ranges are typicallyrepresented on a compressor map, which identifies envelopes of operationfor various levels of efficiency, as well as limits past which thecompressor will have failure conditions such as surge or choke. A rapidincrease in engine 101 backpressure could cause a surge condition, whichis characterized by a pulsating backflow through the compressor and aviolent vibration of the system.

Without any assistance, a transition between two far-apart power levelscould be accomplished using a very gradual transition between therelated fuel-flow rates. However, such a slow transition would not beconducive to providing a quick response to changing power needs.

To better provide for transition between power levels, the controlsystem 113 controls the rate at which fuel flows to the engine 101 andthe rate at which the engine rotates, the rate at which fuel flows tothe afterburner injector 139 of the afterburner 137, and the rate atwhich coolant flows to each cooler (i.e., the intercooler 125 and theaftercooler 135). Alternatively (or in conjunction), other controlmechanisms such as turbocharger wastegates and various system blow-offvalves and bypasses can be controllably employed by the control systemto maintain the system components in efficient operating ranges whilerapidly transitioning between fuel-flow levels.

As will be described below, the control system 113 is configured (e.g.,it is a computer programmed to transmit control signals) to provide forefficient transition between powerplant power generation levels using avariety of control parameters. It is also within the scope of theinvention for the control system to control less than all of theseparameters. For example, an alternative embodiment of the inventioncould have a control system configured to only control the rate at whichfuel flows to the engine 101 and the rate at which fuel flows to theafterburner 137.

Afterburner

A catalyst may be used to readily react hydrogen with oxygen. No flameis required to maintain the reaction, and such a catalytic burner canoperate over a very wide range of equivalence ratios (i.e., the ratio ofthe hydrogen flow to the hydrogen flow that would be needed to fullyreact with all of the oxygen present). Operated in a hot exhaust stream,a catalyst bed of a catalytic burner will react even extremely smallamounts of hydrogen. There is no concern about keeping a stable flamefront or a flame blowing out. In an exhaust stream containing oxygen,any added hydrogen will react when it passes through a catalytic burner.Nevertheless, the maximum amount of hydrogen added must be limited inorder to keep the catalyst bed and turbine below their maximum operatingtemperatures.

In one aspect of the invention, the control system 113 is configured tocontrol the operation of one or both turbochargers to improve engineefficiency during a transition in generated power levels. Moreparticularly, when a rapidly increasing engine fuel-flow rate coulddrive the engine 101 into a less efficient (or an undesirable)equivalence ratio (e.g., a ratio causing an engine misfire or overtemperature event), the control system commands the afterburner fuelinjector 139 to inject fuel into the afterburner 137 prior to and/orduring the increase in the engine fuel-flow rate. The resulting increasein exhaust stream energy increases the operation (i.e., rotation rate)of both turbochargers, and particularly of the second-stageturbocharger. The increased turbocharger rotation rate provides for acompression boost, i.e., an increased flow rate of the gaseous secondreactant (e.g., of pressurized air) to the inlet 105, and thus providesfor a better equivalence ratio during the engine fuel-flow ratetransition.

Likewise, when a rapidly decreasing engine fuel-flow rate could drivethe engine 101 into a less efficient (or an undesirable) equivalenceratio (e.g., a ratio causing the engine to run too lean), and if theafterburner fuel injector 139 is already injecting fuel (such as mightoccur during takeoff or other conditions characterized by high powerrequirements), the control system 113 commands the afterburner fuelinjector to inject less fuel (or no fuel) into the afterburner 137 priorto and/or during the decrease in the engine fuel-flow rate. Theresulting decrease in exhaust stream energy decreases the operation(i.e., rotation rate) of both turbochargers. The decreased turbochargerrotation rate provides for a decreased flow rate of the gaseous secondreactant (e.g., of pressurized air) to the inlet 105, and thus providesfor a better equivalence ratio during the engine fuel-flow ratetransition. Thus, the control system is configured to control theoperation of the turbochargers by controlling the amount of additionalreactants reacted in the exhaust stream.

In either case, once the engine fuel-flow rate reaches a steady statecondition, the system will typically have an appropriate (i.e.,increased or decreased, respectively) energy level available in theexhaust stream of the engine 101 to support the turbochargers at the newoperation levels (usually without additional fuel being supplied to theafterburner 137). Thus, the operation of the afterburner can be used tospeed a change in turbocharger compression rates, and thereby increasethe response efficiency (i.e., the response time while maintaining anefficient, safe and functional operating condition) with which thepowerplant can respond to a change in power requirements.

Intercooler

In another aspect of the invention, the control system 113 is alsoconfigured to control the operation of the turbocharger to improveengine 101 efficiency, both during steady state operation and during atransition in generated power levels, but this time by a differentmethod. As noted above, the efficient operation of a compressor isrelated to the corrected air flow, i.e., the corrected mass flow rate ofair, taking into account air density (ambient temperature and pressure).Thus, by adjusting the temperature of the air being compressed, theefficiency of the compressor can be controlled, and more importantly,can be maintained within safe operation limits.

With regard to control over the rate at which coolant flows to theintercooler, the control system 113 monitors the operation of thesecond-stage turbocharger using sensors typically known for monitoringturbocharger operation. These turbocharger operation sensors and relatedprocessing will provide a necessary set of parameters for determiningwhere the second-stage compressor is operating on its compressor map,e.g., the compressor rotation rate, the compression ratio, the airtemperature, and other related parameters.

In response to changes in powerplant operating conditions, e.g., changesin power generation requirements and/or in altitude, the control system113 commands the rate of coolant delivered to the intercooler 125 to beincreased or decreased in order to change the corrected air flow rate tothe second stage compressor, and thereby drive the compressor operatingpoint to a place that improves and preferably maximizes the powerplantsystem efficiency. It should be noted that it is powerplant efficiencyis the goal of the changes in coolant delivery. Turbocharger efficiencymay likewise be improved, but it is not necessarily so.

Aftercooler

The efficient operation of the powerplant is closely related to theengine equivalence ratio, i.e., the fuel to air ratio over the fuel toair ratio that would provide for complete reaction of all the fuel andoxidizer. By adjusting the temperature of the gaseous oxidizer stream(the twice-compressed and cooled air stream) at the inlet 105, thedensity of the stream can be changed, and the amount of air being mixedwith each quantity of fuel of a reciprocating engine, can be adjusted.

The operation of the aftercooler 135 can be controlled by the controlsystem to selectively change the engine 101 inlet air temperature. Thecontrol system 113 is thereby configured to control the operation of theaftercooler to improve, and preferably maximize, engine efficiency atall required operating conditions.

Method of Generating Power

With reference to FIGS. 1 & 4, using the above-described combustionengine 101 configured to produce motive force, and using the primarygenerator 103 configured to generate power from the motive force of theengine, the invention provides a method of generating power. The methodincludes the step of controlling 171 the motive force applied to theengine by the primary generator such that the primary generatormaintains the engine equivalence ratio in an appropriate range tomaximize powerplant efficiency while keeping all components within theirthermal and mechanical operating limits.

As noted above, the primary generator 103 generates power at a powergeneration level related to a rate at which fuel is provided to theengine 101. The method includes the step of varying 173 the rate atwhich fuel is provided to the engine in order to change the powergeneration in response to varied power needs.

In further using at least one of the above-described turbines 123, 133driven by the exhaust stream of the engine 101, at least one relatedcompressor 121, 131, which is configured to compress gaseous engineoxidizer for the engine, and the above-described afterburner 137intermediate the engine and the turbines, the method further includesthe step of controlling 175 the amount of additional reactants reactedin the exhaust stream to maximize powerplant efficiency while keepingall components within their thermal and mechanical limits.

Operating Temperature Limits

With reference to FIGS. 1 & 5, some hydrogen internal combustion engineshave a propensity to back-flash or backfire if their exhaust gets toohot. Also, the catalyst beds of catalytic burners have temperaturelimits. Under an aspect of the present invention, during very high poweroperation the engine 101 is operated leaner than would otherwise beoptimal (e.g., leaner than would be typical at moderate power) in orderto maintain the engine exhaust temperature below a predefined limit. Thethird reactant, (the hydrogen fuel) is injected into the exhaust by theafterburner injector 139, and reacted in the afterburner 137. Thisachieves a higher turbine energy extraction, and thus a highercompressor boost pressure, and results in a higher gaseous reactant flowrate that maintains a leaner engine operation and a lower engine exhausttemperature.

More particularly, the control system 113 is configured to monitor andrespond to the exhaust temperature of the engine 101, either by directlymeasuring that exhaust temperature (one aspect of the operatingcondition of the engine), or by monitoring other aspects of theoperating condition of the engine that are indicative of exhausttemperature. In the former case, in order to monitor the exhausttemperature, the powerplant is equipped with a temperature sensor 151positioned and configured to sense the temperature of the engine exhauststream from the exhaust outlet 107. In alternative embodiments, thesystem controller 113 is provided with information allowing it identifyoperating conditions during which the exhaust temperature needs to belowered. Such relevant information may include engine fuel injectionrates, engine cooling system activity, and the like.

As previously recited, in response to a given power generationrequirement, a required fuel-flow rate for meeting the requirement isidentified by the control system 113. The control system transitions theengine 101 to operate at the desired fuel-flow rate (i.e., it sendscontrol signals to the engine fuel injection system and other systems asdescribed above to transition to fuel flowing to the engine at theidentified fuel-flow rate). The control system is configured to controlthe flow rate that the gaseous stream of second reactant is provided tothe engine, based on the operating condition of the engine, such thatthe exhaust stream temperature is maintained below a temperature limit.

In the case where the control system 113 monitors the actual exhausttemperature, the control system 113 is configured to control the flowrate that the gaseous stream of second reactant is provided to theengine 101 based on the exhaust stream temperature sensed by thetemperature sensor 151, such that the sensed exhaust stream temperatureis maintained below the temperature limit. Thus, based on the sensedexhaust stream temperature, (i.e., in response to an engine exhausttemperature that approaches or exceeds a limit value), or alternativelyin response to engine operating conditions that are understood toproduce temperatures that approach or exceed the limit value, thecontrol system transmits control signals causing the flow rate of thegaseous stream of second reactant to be increased such that the exhauststream temperature is maintained below the temperature limit.

In the present embodiment, the control system 113 is configured tocontrol the flow rate of the gaseous stream of second reactant bytransmitting control signals that control the operation of thecompressors 121, 131, and/or that control the operation of the coolers125, 135 (and particularly the aftercooler 135). The control system isfurther configured to control the operation of the compressors by havingthe control signals control the driving force from the turbines 123, 133to drive the compressors.

To implement this, the control system 113 of the embodiment isconfigured to control the driving force from the turbines 123, 133 byhaving the control signals control the amount of additional reactantsthat are injected by the afterburner injector 139 into the exhauststream and reacted by the catalytic afterburner 137. The temperaturelimit of the present embodiment is a constant, but the broadest scope ofthe invention is understood to include temperature limits that arefunctions of various parameters, such as altitude.

This aspect of the invention includes a related method of limiting thetemperature of an exhaust stream of an engine 101. This method includesthe step of monitoring 181 information indicative of an exhaust streamtemperature, and the step of controlling 183 the flow rate of thegaseous stream of second reactant based on the monitored informationsuch that the exhaust stream temperature is maintained below atemperature limit. The step of monitoring may include the step ofsensing 185 an exhaust stream temperature, and the step of controllingmay comprise controlling the flow rate of the gaseous stream of secondreactant based on the sensed exhaust stream temperature such that theexhaust stream temperature is maintained below a temperature limit.

In the case where the engine 101 is provided with a compressor, aturbine, a catalytic afterburner 137, and an afterburner injector, ashave been described above, in the step of controlling, the flow rate ofthe gaseous stream of second reactant may be controlled by controlling187 the quantity of the third reactant injected into the exhaust streamby the afterburner injector.

Engine Restart at Altitude

When an aircraft powerplant becomes shut down while flying at highaltitudes, there are potential difficulties that may limit its abilityto restart. Even though the primary generator 103 may be able to turnthe engine 101 at reasonably high speed, the airflow through the engineduring engine cranking will be very low due to low outside air density.To meet the necessary air-fuel ratio for combustion this small airflowwould correspondingly require a very small hydrogen flow rate.

Nevertheless, fuel injectors may have a minimum hydrogen flow rate thatis too high to achieve the correct mixture with such low airflow.Moreover, a certain level of combustion will be necessary for enginestartup. Thus, a powerplant that is at a very high altitude may be facedwith a significant requirement for oxidizer compression to meet both thenecessary air-fuel ratio for combustion and the minimum fuel flownecessitated by the engine configuration.

With reference to FIGS. 1 & 6, under another aspect of the inventionapplicable to this embodiment, the engine 101 is cranked at a highspeed, and hydrogen fuel is added to the exhaust by the afterburnerinjector 139 and reacted in the afterburner 137 before introducinghydrogen fuel into the engine fuel injectors. The heat added to theexhaust by the afterburner adds energy to the turbines 123, 133, andthus powers the air compression system to bring up at least a smalllevel of compression boost. This boost increases airflow through theengine, which provides sufficient airflow to meet the requirement foroxidizer compression based on the necessary air-fuel ratio forcombustion and the minimum fuel flow necessitated by the engineconfiguration.

In the embodiment, the above-described engine 101, which includes one ormore compressors 121, 131, one or more related turbines 123, 133(forming turbochargers with the compressors), an afterburner 137 and acontrol system 113, is configured for use in a range of ambientconditions. The control system is configured to restart the engine in atleast some ambient conditions of the range of ambient conditions bycontrolling the operation of the afterburner to power the turbine(s)such that the compressor(s) are driven at a speed that provides for thegaseous stream of second reactant to be provided to the engine at a flowrate at or above a threshold level. This threshold level may be aconstant, or it may be a function dependant upon altitude, temperature,and other such parameters.

Under this aspect of the invention, the control system 113 may controlthe afterburner 137 by transmitting control signals to an afterburnerinjector 139 configured to inject a third reactant (e.g., hydrogen fuel)in the exhaust stream for reaction by the afterburner. The controlsystem is configured to control the amount of additional reactantsreacted in the exhaust stream by controlling the quantity of the thirdreactant injected into the exhaust stream by the afterburner injector.

This aspect of the invention further includes a related method ofrestarting an engine 101. As previously stated, the engine is configuredto react a gaseous stream of second reactant (e.g., compressed air),which is provided to the engine at a flow rate, with a second reactant(e.g., a hydrogen fuel) to produce energy and an exhaust stream, and isconfigured for use in a range of ambient conditions.

Under the method, an afterburner 137 that reacts reactants in theexhaust stream intermediate the engine 101 and the turbine is operated191 during a restart of the engine. This is done such that a compressoris driven by a turbine at a speed that provides for the gaseous streamof second reactant to be provided to the engine at a flow rate at orabove a threshold level. The afterburner is operated by the controlsystem 113 transmitting control signals causing an injection 193 of acontrolled amount 195 of a third reactant (e.g., a hydrogen fuel) in theexhaust stream for reaction by the afterburner.

Alternative Power Recovery

It is anticipated that in a typical hydrogen internal combustion engine,only about half of the oxygen is burned. That means that half the energyused in compression is not recovered in combustion. Another embodimentof the invention provides for the recovery of some of this energy, andprovides for further synergies with other system operation needs.

With reference to FIG. 7, a second embodiment of the invention isidentical to the first (using reference numbers incremented by 100 forlike components), with the exception of certain additional components,and with the control system being configured to advantageously operatethe components. More particularly, the second embodiment may be providedwith an internal combustion piston engine 201 that drives a primarymotor-generator 203 (“the primary generator”), a control system 213, aram-air scoop 219, a first-stage turbocharger including a first-stagecompressor 221 and a first-stage turbine 223, an intercooler 225, asecond-stage turbocharger including a second-stage compressor 231 and asecond-stage turbine 233, an aftercooler 235, an afterburner 237, atemperature sensor 251, a low-pressure wastegate 261, a high-pressurewastegate 263, an engine bypass 265 and an inlet blow-off valve 267.

The embodiment further includes a first turbine generator 272 that isdriven by the shaft that connects the first-stage turbine 221 andfirst-stage compressor 223, and a second turbine generator 274 that isdriven by the shaft that connects the second-stage turbine 231 andsecond-stage compressor 233. By using the afterburner 237 to burn someof the remaining oxygen in the exhaust and increase the exhausttemperature, an excess of energy is made available via the expansion ofthe exhaust through the turbines over the level that is needed to powerthe air compression system. This energy is recovered using theseelectric turbine generators. In this embodiment, the control system isconfigured to control the injection of fuel into the afterburner at arate to maximize the energy efficiency of the powerplant.

It is anticipated that so the overall inefficiency of recovering theadded energy may be in the 30-40% range. Alternative embodiments may beconfigured with only one of the two described turbine generators, orwith the addition of a separate turbine and turbine generator in theexhaust stream.

In a variation of this embodiment, the turbine generators can beconfigured to controllably generate a variable level of power (and causea resulting motive force to be applied to their related turbines), suchas by having field coils that can be energized to a variety of levels.The control system is configured to control the motive force applied bythe turbine generators to their respective turbines such that theturbine generators only remove excess power, but allow for thecompressors to operate at their most efficient levels given the currentconditions.

The controlled extraction of power by the turbine generators providesfor additional synergies in light of the various functions describedabove for the afterburner and coolers. For example, when a rapidlydecreasing power requirement causes the engine fuel-flow rate to rapidlydecrease, the control system increases the power generation levels ofthe turbine generators (i.e., the motive force with which they resistthe rotation of the turbines). This causes a decrease in the speed ofthe turbochargers, and a rapid reduction in the rate of compression bythe compressors, compensating for an increasing pressure ratio acrossthe compressor and avoiding an approach to a surge condition (similar tothat discussed above regarding intercooler activity).

Similarly, if the turbine generators are actively generating electricityand a rapidly increasing power requirement causes the engine fuel-flowrate to rapidly increase, the control system decreases the powergeneration levels of the turbine generators (i.e., the motive force withwhich they resist the rotation of the turbines). This causes a rapidincrease in the speed of the turbochargers, and an increase in the rateof compression by the compressors, avoiding an impending approach to achoke condition (again, similar to the discussion above regardingintercooler activity).

Thus, the operation of the turbine generators can be controlled tomaintain high efficiency levels in the first- and second-stagecompressor compression rates, and thus increase the efficiency (i.e.,the response time while maintaining an efficient, safe and functionaloperating condition) with which the powerplant can respond to a changein power requirements. The control system is configured to control theoperation of the turbine generators to improve engine efficiency duringa transition in generated power levels.

Furthermore, the turbine generators can optionally be configured tooperate as motors. Thus, when an increase in turbocharger speed isneeded (either on startup or when the fuel-flow rate is increasing),electrical energy can be applied to the turbine generators and thecompressors can operate at levels higher than would otherwise beavailable.

Fuel Cell Variant

With respect to FIG. 8, in some variations of the present invention, thepowerplant may comprise a power generation system in the form of a fuelcell 301, rather than an engine and a primary generator. With the fuelcell, a compression system may still be needed to compress a gaseousstream of reactant (e.g., an oxidizer such as air). For a fuel cellsystem, the exhaust may still have around half of the original oxygencontent of the inlet air. Therefore, in a configuration similar to thosepreviously disclosed, an afterburner 337 can be used in the exhauststream to power one or two turbochargers that provide for thecompression needed to run the fuel cell. An intercooler 325 and anaftercooler 335 can likewise be used, similar to the way described abovefor an internal combustion engine with a generator.

Thus, an aspect of the present invention contemplates a fuel cell powergeneration system configured to react a first reactant with a gaseousstream of second reactant to produce energy and an exhaust stream, alongwith a first-stage compressor 321, a second-stage compressor 331, afirst-stage turbine 323, a second-stage turbine 333, an afterburner 337,and a control system (not shown, but substantially similar to thosedescribed above). The compressors are configured to pressurize thegaseous stream of second reactant supplied to the power generationsystem.

The compressors 321, 331 are driven in rotation by a driving force fromthe turbines 323, 333, which are propelled by the exhaust stream. Theafterburner 337 is located in the exhaust stream intermediate the powergeneration system and the turbines, and is configured to react reactantsin the exhaust stream. The control system is configured control theoperation of the afterburner. It does so at a rate such that thecompressors are driven at a speed that provides for the gaseous streamof second reactant to be provided to the power generation system at aflow rate appropriate to a rate at which the first reactant is beingprovided to the power generation system.

The system also has a component for initial startup airflow to feed theafterburner. This component may either be a motor generator on one orboth turbines (not shown, but shown and described for previousembodiments), or a compressed air start system.

It is to be understood that the invention comprises apparatus andmethods for designing powerplants, and for producing powerplants, aswell as the apparatus and methods of the powerplant itself.Additionally, the various embodiments of the invention can incorporatevarious combinations of the above-described features. Moreover, it iscontemplated that the claims are broader than the described embodiment.

For example, the above embodiments all recited that the second gaseousreactant was an oxidizer, that the first and third reactants were of thesame type (a fuel) and the same composition (hydrogen), and that theequivalence ratio was less than one, leaving excess oxidizer in theexhaust. It is well within the invention to have the system compress andcontrol the flow of gaseous fuels, to use different fuels, to use aequivalence ratio of greater than one, and/or to add an oxidizer as thethird reactant. In short, the above disclosed features should not undulylimit the claims, and can be combined in a wide variety ofconfigurations within the anticipated scope of the invention.

While particular forms of the invention have been illustrated anddescribed, it will be apparent that various modifications can be madewithout departing from the spirit and scope of the invention. Thus,although the invention has been described in detail with reference onlyto the preferred embodiments, those having ordinary skill in the artwill appreciate that various modifications can be made without departingfrom the scope of the invention. Accordingly, the invention is notintended to be limited by the above discussion, and is defined withreference to the following claims.

1. A powerplant, comprising: a combustion engine configured to producemotive force, and having an inlet and an outlet, and being characterizedat any given time by an engine equivalence ratio; a generator configuredto generate power from the motive force of the engine, and configured toapply a variable-level motive force to the engine; and a control systemconfigured to control the motive force applied to the engine by thegenerator, wherein the control system is configured to control thegenerator such that the generator maintains the engine speed based upona calculation of the engine equivalence ratio.
 2. The powerplant ofclaim 1, wherein the engine is configured with a passageway connectingan engine inlet to a source of gaseous reactant, and wherein thepassageway is configured to pass a stream of gaseous reactant from thesource of gaseous reactant without limitation from a controllableobstruction that would cause a reduction in pressure of the stream ofgaseous reactant.
 3. The powerplant of claim 2, and further comprising acompressor configured to compress the stream of gaseous reactant fromthe source of gaseous reactant, wherein the passageway is configured toalways provide the stream of gaseous reactant to the engine inlet atsubstantially the pressure level established by the compressor.
 4. Thepowerplant of claim 2, wherein the engine is a hydrogen-fueled engineconfigured to run with an equivalence ratio of less than one, and thestream of gaseous reactant is an oxidizer.
 5. The powerplant of claim 1,wherein the generator generates power at a power generation levelrelated to a rate at which fuel is provided to the engine, and whereinthe control system is configured to change the power generation level byvarying the rate at which fuel is provided to the engine.
 6. Thepowerplant of claim 5, and further comprising a turbocharger including aturbine driven by an exhaust stream of the engine and a compressorconfigured to compress stream of gaseous reactant for the engine,wherein the control system is configured to control the operation of theturbocharger based on a calculation of an efficiency level of thepowerplant.
 7. The powerplant of claim 6, and further comprising anafterburner in the exhaust stream intermediate the engine and theturbine, the afterburner being configured to react additional reactantsin the exhaust stream, wherein the control system is configured tocontrol the operation of the turbocharger by controlling the amount ofadditional reactants reacted in the exhaust stream.
 8. The powerplant ofclaim 7, and further comprising a second turbocharger including aturbine driven by an exhaust stream of the engine and a compressorconfigured to compress stream of gaseous reactant for the engine,wherein the second turbocharger turbine is upstream of the afterburner.9. The powerplant of claim 6, wherein the control system is configuredto control the operation of the turbocharger such that the calculationof the efficiency level of the powerplant is maximized at all requiredpower levels and ambient operating conditions.
 10. The powerplant ofclaim 5, and further comprising a compressor configured to compress thestream of gaseous reactant for the engine, and an aftercooler configuredto cool the stream of gaseous reactant compressed by the compressor,wherein the control system is configured to control the operation of theaftercooler based on a calculation of an efficiency level of thepowerplant.
 11. The powerplant of claim 10, wherein the control systemis configured to control the operation of the aftercooler such that thecalculation of the efficiency level of the powerplant is maximized atall required power levels and ambient operating conditions.
 12. Thepowerplant of claim 5, and further comprising two serial compressorsconfigured to compress gaseous reactant for the engine, and anintercooler configured to cool the gaseous reactant intermediate the twocompressors, wherein the control system is configured to control theoperation of the intercooler based on a calculation of an efficiencylevel of the powerplant.
 13. The powerplant of claim 12, wherein thecontrol system is configured to control the operation of the intercoolersuch that the calculation of the efficiency level of the powerplant ismaximized at all required power levels and ambient operating conditions.14. A method of generating power using powerplant including a combustionengine configured to produce motive force, the engine having an inletand an outlet, and the engine being characterized at any given time byan engine equivalence ratio, and further including a generatorconfigured to generate power from the motive force of the engine, thegenerator being configured to apply motive force to the engine whenelectrical power is applied to the generator, comprising: controllingthe motive force applied to the engine by the generator such that thegenerator maintains the engine speed based upon a calculation of theengine equivalence ratio.
 15. The method of claim 14, wherein thegenerator generates power at a power generation level related to a rateat which fuel is provided to the engine, and further comprising changingthe power generation in response to varied power needs by varying therate at which fuel is provided to the engine.
 16. The method of claim15, wherein the powerplant includes a turbocharger including a turbinedriven by an exhaust stream of the engine and a compressor configured tocompress a stream of gaseous reactant for the engine, and furtherincludes an afterburner in the exhaust stream intermediate the engineand the turbine, the afterburner being configured to react additionalreactants in the exhaust stream, and further comprising controlling theamount of additional reactants reacted in the exhaust stream based on acalculation of an efficiency level of the powerplant.
 17. A powerplant,comprising: a combustion engine configured to produce motive force, andhaving an inlet and an outlet; a generator configured to generate powerfrom the motive force of the engine, and configured to apply motiveforce to the engine when electrical power is applied to the generator;and a means for controlling the motive force applied to the engine bythe generator such that the generator maintains the engine speed basedupon a calculation of the engine equivalence ratio.
 18. The powerplantof claim 17, wherein the generator is configured to generate power at apower generation level related to a rate at which fuel is provided tothe engine, and further comprising a means for changing the powergeneration in response to varied power needs by varying the rate atwhich fuel is provided to the engine.
 19. The powerplant of claim 18,wherein the engine includes a turbine driven by an exhaust stream of theengine, and a compressor configured to compress a stream of gaseousreactant for the engine, and an afterburner in the exhaust streamintermediate the engine and the turbine, the afterburner beingconfigured to react additional reactants in the exhaust stream, andfurther comprising a means for controlling the amount of additionalreactants reacted in the exhaust stream based on a calculation of anefficiency level of the powerplant.